1. Field of the Invention
The present invention generally relates to zoom lenses and, more particularly, is directed to a zoom lens for use with electronic still cameras, video cameras or the like, for example.
2. Description of the Related Art
At present, consumer video cameras employ a lens having a zooming ratio of 6 times or more and whose F number is smaller than 2.0 as a zoom lens. Recently, it has become desirable for the video camera to be miniaturized and reduced in weight while maintaining the features such that the zoom lens has large variable magnification and which also is bright.
To meet the requirements such that the video camera is miniaturized and reduced in weight, an imager size of a charge-coupled device (CCD) image pickup device utilized as the image pickup element is miniaturized more because it is to be noted from a scaling law standpoint that a camera lens itself can be miniaturized in accordance with the reduction of the imager size.
The conventional zoom lens, however, comprises 10 spherical lenses or more. There is then the disadvantage such that the conventional lens cannot be miniaturized and reduced in weight sufficiently. Further, when the camera lens is miniaturized from a scaling law standpoint, the resulting precison required for each of the lenses constituting the zoom lens also is increased so that, if the number of the lenses constituting the zoom lens remains the same, then the zoom lens cannot be processed without difficulty. Particularly, when a lens element is finished as a spherical surface by the polishing-treatment, if the radius of curvature of the spherical surface is reduced, there is then the risk that a manufacturing cost for achieving the required precision prescribed is increased considerably.
In order to remove the above-mentioned disadvantage and risk, a method of manufacturing a lens by molding some suitable materials such as plastics, glass or the like has been established recently. According to this method of manufacturing the lens by the molding-process, there is then the possibility that upon mass-production the manufacturing cost of the lens can be reduced as compared with the above-mentioned method of manufacturing the lens by the polishing-process. Further, in accordance with the molding-process, the lens surface can be formed as an aspherical surface with ease and the number of lens elements constructing the camera lens can be reduced, which can make a great contribution to the camera lens when the camera lens is miniaturized and reduced in weight.